This invention relates to an epoxy resin crosslinked polyester powder coating. More particularly, this invention relates to the modification of the polyester typically used in carboxyl functional polyester-epoxy coatings with 1,3-propanediol (1,3-PDO) for significantly improved flexibility and impact resistance, with other key properties unchanged over a broad range of 1,3-PDO concentrations. In addition, the 1,3-PDO modified polyester has lower melt viscosity than polyesters made with 100% neopentyl glycol.
The development of powder coatings has become increasingly significant in recent years. Powder coatings release no harmful solvents during application, may be applied in a highly efficient manner with little waste, and, thus, are considered particularly environmentally friendly and economical. Usable powder coatings may be obtained using curable epoxy resin powder coatings. Many epoxy resin powder coatings currently commercially available contain solid polyester polyols which are cured with epoxy resins.
Polymers used in the manufacture of powder coatings are classified broadly as either thermosetting or thermoplastic. Thermosetting coatings, when compared to coatings derived from thermoplastic compositions, generally are tougher, more resistant to solvents and detergents, have better adhesion to metal substrates, and do not soften when exposed to elevated temperatures. However, the curing of thermosetting coatings has created problems in obtaining coatings which have, in addition to the above-stated desirable characteristics, good smoothness, and flexibility.
Thermoset powder coatings can be grouped into epoxy, epoxy-polyester, polyester-urethane, TGIC polyester, and acrylic. Epoxy-polyester coatings have better color retention and UV resistance than epoxy powder coatings. Such coatings are widely used in the household equipment such as refrigerators, freezers, washing machines, stoves, and other kitchen equipment, metal furniture, ceiling panels for the building industry, shower cabinets, automotive components, agricultural equipment, and machinery, engineering, and electrical parts.
Coatings derived from thermosetting coating compositions should possess good impact strength, hardness, flexibility, and resistance to solvents and chemicals. For example, good flexibility is essential for powder coating compositions used to coat sheet (coil) steel which is destined to be formed or shaped into articles used in the manufacture of various household appliances and automobiles where the sheet metal is flexed or bent at various angles.
Powder coating systems based on carboxyl polyesters such as neopentyl glycol based carboxyl polyesters and melamine-, benzoguanamine-, and urea-formaldehyde cross-linking agents have been used in the coatings industry. See, for example, those described in U.S. Pat. No. 5,739,204, incorporated by reference herein in the entirety. EP 8344 discloses a similar system and mentions the use of 1,3-PDO.
Neopentyl glycol is often used in formulations for polyester powder coatings along with a mixture of terephthalic and isophthalic acids either by themselves or as mixtures and optionally branched with small amounts of trimethylol propane. Typically when the polyester used in this type of powder coating is modified for improved flexibility, impact, and toughness, then other properties of significance to the coating are compromised. Other potential modifiers include adipic acid, 1,4-butanediol, and 2-methyl-1,3-propanediol.
There does not appear to be any reference in the art which suggests the incorporation of 1,3-propanediol into a carboxyl functional polyester epoxy resin powder coating composition for increased impact resistance and flexibility without the loss of other critical properties and also where the 1,3-PDO modified polyester has lower melt viscosity than polyesters made with 100% neopentyl glycol.
In accordance with the foregoing, the present invention comprises a polyester epoxy resin powder coating exhibiting improved impact resistance and flexibility, without the loss of other key properties and where the 1,3-PDO modified polyester has lower melt viscosity than polyesters made with 100% neopentyl glycol, which is formed by reacting:
a) A carboxyl functional polyester resin formed by reacting one or more aliphatic glycols and one or more polycarboxylic acids and/or anhydrides, wherein 5 to 90% (on a molar basis) of the aliphatic glycol is 1,3-propanediol, in the presence of an esterification catalyst and then endcapping the polyester with an endcapping agent to ensure that the polyester has carboxyl chain ends; and
b) An epoxy resin crosslinking agent.
It has now been demonstrated that by partially replacing the neopentyl glycol with 1,3-propanediol the formulation exhibits improved impact resistance and flexibility with other key properties essentially unchanged over a broad range of 1,3-propanediol concentrations and also that the 1,3-PDO modified polyester has a lower melt viscosity than polyesters made with 100% neopentyl glycol.